The present invention relates to dispersants for hydrophobic particles and particulate including pigments in water-based systems. In particular, the inventive dispersant can be used for pigments such as those useful as inks adapted for ink jet printing applications.
When hydrophobic particles are placed in water they tend to agglomerate. This tendency makes it difficult to obtain a dispersion of hydrophobic particles in a water-based system. To lessen this agglomeration tendency, a dispersant may be added to the mixture. If the dispersant has both hydrophobic and hydrophilic parts, the hydrophobic part will be attracted to the hydrophobic particle surface and the hydrophilic part will be solvated by the water.
In recent years, the use of thermal ink jet printers in numerous applications has increased dramatically. Such printers make use of liquid-based inks which are sprayed onto a receptor, typically a sheet of paper or film, to produce an image. By using four basic ink colors (black, yellow, cyan, and magenta or black, red, blue and yellow) as well as white pigments (such as TiO2) in various combinations and concentrations, virtually any color may be produced as part of the printed image. Additionally, ink jet technology is well-suited for high resolution graphic images, particularly those produced using electronic printing systems. Such systems typically employ computer technology to create, modify, and store images, text, graphics and the like.
Many of the inks that have been used in the past with ink jet and other printers are primarily comprised of dyes contained within organic-based carrier liquids. Although such inks may offer satisfactory performance in certain applications, the present trend is away from such systems, because such systems may tend to produce images that lack the light stability and durability required for outdoor and similarly demanding applications. Additionally, the use of organic-based carrier liquids may involve environmental and material-handling restrictions. The printing industry has sought inks that are primarily water-based to thereby reduce or eliminate restrictions associated with organic solvent-based systems.
Inks may comprise a suspension of pigment particles in a water-based carrier. However, the suspended pigments may tend to agglomerate. Because ink jet printers use very small jet nozzles (on the order of less than about 40 micrometers with droplet volumes on the order of picoliters) to provide high resolution images, pigment agglomerations may restrict or clog the printer heads. This effect is referred to herein as xe2x80x9cplugging.xe2x80x9d Additionally, in the case of thermal ink jet systems, the ink is subjected to high temperatures (approximately 350xc2x0 C.), as a result of the heating element in the nozzle. Typically inks tend to agglomerate faster at higher temperatures, and may also settle onto, and coat, the heating elements of the printer heads. This causes a decreased thermal efficiency of the print head, which results in the formation of smaller ink droplets and lower image quality. This effect is commonly referred to as xe2x80x9ckogation.xe2x80x9d
To overcome the problems described above, pigment particles in some water-based ink jet inks have been stabilized from agglomeration with dispersants. In one approach, the dispersants were formed from surfactants having a hydrophilic portion as well as a hydrophobic portion, which hydrophobic portion adsorbs to pigment surfaces of varying hydrophobicity. In another approach, copolymers having hydrophilic segments and hydrophobic segments were used. Examples of these approaches are described in U.S. Pat. No. 4,597,794 and U.S. Pat. No. 5,085,698.
In the approaches described above, the hydrophobic segments of the surfactant or polymer can adsorb onto pigments by hydrophobic interaction between the dispersant molecule and the organic-based pigments whose surfaces tend to be hydrophobic. This hydrophobic interaction is usually not very strong. Because of this weak attraction, in thermal ink jet systems it is possible that the dispersant molecules can desorb from pigment surfaces, thereby allowing the pigment particles to agglomerate. During printing, this can result in plugging of the printer head jet nozzles. Although heat produced in a thermal ink jet system may strengthen the adsorption of the dispersant on the hydrophobic pigment surface, thus enhancing the stability of the ink, plugging and kogation remain a problem.
In view of the foregoing, a need exists for dispersants that strongly adsorb to hydrophobic particles and that inhibit particle agglomeration. A need also exists for water-based inks that offer increased stability. The present invention addresses such needs.
One aspect of the invention provides a dispersant for dispersing hydrophobic particles in an aqueous system comprising a hydrophobic segment comprised of a nonpolymerized hydrocarbon moiety joined to at least one terminal polymeric hydrophilic segment comprised of a polyelectrolyte.
Another embodiment of the invention relates to stable water-based systems in which an inventive dispersant is incorporated into a water-based dispersion of particles, the particles having hydrophobic surfaces. Such systems comprise, generally, an aqueous suspension of a) hydrophobic particles, and b) a dispersant comprising a hydrophobic segment comprised of a nonpolymerized hydrocarbon moiety, which can adsorb to hydrophobic particle surfaces, joined to at least one terminal polymeric hydrophilic segment comprised of a polyelectrolyte.
An alternate embodiment of the invention provides a dispersant wherein the hydrocarbon moiety has two opposite ends and wherein a polyelectrolyte is attached to each end of the hydrocarbon moiety.
Another embodiment of the invention provides a mixture of linear hydrophobe-terminated hydrophilic copolymers and multibranched hydrophilic copolymers.
A particular application of the present invention relates to stable water-based pigmented inks that meet the stringent requirements of ink jet and other printing applications. In these inks, an inventive dispersant is incorporated into a water-based pigment dispersion to inhibit agglomeration of the hydrophobic pigment particles.
As used in this invention:
xe2x80x9cadsorbxe2x80x9d refers to the adherence of an atom, ion, or molecule to the surface of another substance;
xe2x80x9cdispersant/particle compositexe2x80x9d means a dispersant molecule and the particle or particulate to which the dispersant is adsorbed; typically the composite will comprise layers of dispersant molecules on the particle or particulate;
xe2x80x9caliphatic groupxe2x80x9d or xe2x80x9caliphatic moietyxe2x80x9d xe2x80x9clong chain aliphatic group/moietyxe2x80x9d means a linear, branched, cyclic, substituted or unsubstituted, non-polar hydrocarbon having approximately 6 to 100, preferably 8 to 20, carbon atoms;
xe2x80x9caromatic groupxe2x80x9d or xe2x80x9caromatic moietyxe2x80x9d means a nonpolar hydrocarbon, having approximately 6 to 100, preferably 8 to 20, carbon atoms, comprising at least one cyclic structure containing at least one xcfx80-conjugated ring structure;
xe2x80x9chydrocarbon groupxe2x80x9d or xe2x80x9chydrocarbon moietyxe2x80x9d means an aliphatic or aromatic group; xe2x80x9cbandingxe2x80x9d refers to visible striations or lines of lower optical density that can occur when an ink is printed on a substrate;
xe2x80x9cionizablexe2x80x9d means a neutral molecule comprising atoms capable of losing or gaining electrons, thereby acquiring a net electrical charge and forming an ion, e.g., when in a polar medium such as water;
xe2x80x9cpolyelectrolyte,xe2x80x9d xe2x80x9cpolyelectrolytic,xe2x80x9d xe2x80x9celectrolyticxe2x80x9d means an oligomer, polymer, or copolymer containing ionic constituents wherein an aqueous solution of the oligomer or (co)polymer will conduct an electric current;
xe2x80x9coligomerxe2x80x9d means a polymer having approximately 10 to 50 monomer units;
xe2x80x9crandom copolymerxe2x80x9d means a copolymer comprised of at least two types of monomer units, e.g., A and B, wherein the order of a combination of monomeric units is random;
xe2x80x9chydrophilic segmentxe2x80x9d means the portion(s) of the dispersant, distinct from the hydrophobic portion of the dispersant, that comprises a polymer of water-soluble moieties;
xe2x80x9chydrophobic segmentxe2x80x9d means the portion of the dispersant molecule, distinct from the hydrophilic portion of the dispersant, that is comprised of water-insoluble, nonpolar moieties, which hydrophobic portion can associate or bond with the surface of a hydrophobic particle; the hydrophobic segment is not polymerized, but rather derived from a single aliphatic or aromatic precursor, e.g., octadecyl isocyanate;
xe2x80x9chydrophobic particlexe2x80x9d means a particle that is nonpolar or has a nonpolar surface;
xe2x80x9csolvatexe2x80x9d refers to the phenomenon wherein a strong affinity of a solvent, e.g., water, for particles of dissolved or suspended substances leads to the particles acquiring a tightly held film of solvent on their surfaces;
xe2x80x9cterminalxe2x80x9d means that if the dispersant comprises more than one hydrophilic segment, the hydrophilic segments always constitute end groups of the hydrophobic segment; such structures are exemplified by FIGS. 2 and 3; and
xe2x80x9cwater solublexe2x80x9d refers to a material capable of being homogeneously distributed in water, such as a material that will form an aqueous solution or dispersion.
Further aspects of the invention will become apparent with reference to the following figures and detailed description.